Systems and methods for providing an atc overlay data link

ABSTRACT

Embodiments of the present invention disclose systems and methods for providing an ATC Overlay data link. Through embodiments of the present invention, existing ATC (or other) modulated signals using existing standard frequencies may be utilized to transmit (e.g., from an aircraft transponder) additional information in a manner that does not render the transmitted signal unrecognizable by legacy ATC equipment. Legacy equipment will be able to demodulate and decode information that was encoded in the transmitted signal in accordance with preexisting standard modulation formats, and updated equipment can also extract the additional information that was overlaid on transmitted signals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims full benefit of and priority to U.S. ProvisionalPatent Application No. 60/926,126 filed Apr. 24, 2007 titled, “Systemsand Methods of Providing an ATC Overlay Data Link”, and to U.S.Provisional Patent Application No. 60/931,274, filed May 21, 2007 titled“Systems and Methods of Providing an ATC Overlay Data Link”, thedisclosures of which are fully incorporated herein by reference for allpurposes.

DESCRIPTION OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for encoding andmodulating digital information, and more particularly, to systems andmethods for establishing an alternative data link through multiplemodulation of air-traffic-control-related electronic signals.

2. Background of the Invention

Travel by aircraft is generally a safe and efficient way for travelersto reach remote destinations. Over the years, as the popularity of airtravel has dramatically increased, the need for techniques for safelymanaging the flow of aircraft has also risen. To address air trafficsafety issues, aircraft have been equipped with avionics equipment suchas transponders that assist air traffic controllers in identifying,tracking, and managing aircraft in flight.

Through radio frequency transmissions, transponders provide air trafficcontrollers and other suitably equipped aircraft with information suchas aircraft identification, altitude, and other aircraft-specific data.Ready access to such information allows controllers and pilots toutilize airspace in a safer and more efficient manner. As the density ofair traffic grows, it is understandable that there is a growing need formore information to be relayed between aircraft and ground stations on anear-real-time basis.

Currently, FAA Air Traffic Control and most other ATC controllingauthorities around the world use standard modulation schemes to ensureinteroperability of their radio frequency signals with other aircraftand systems. For example, the Minimum Operational Performance Standardsfor Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S)Airborne equipment, promulgated by RCTA as RTCA/DO-181C (andincorporated by reference herein in its entirety) defines pulse positionmodulation on 1090 MHz for Mode S transponder and older transponder(ATCRBS transponders) replies to 1030 MHz ground station and TCASinterrogations. By using standard protocols aircraft state informationas well as other data can be relayed aircraft to ground, ground toaircraft, or in some instances aircraft to aircraft.

The volume of information that must be transmitted from aircraftcontinues to increase as more advanced avionics and traffic controlsystems become available. Likewise, the need to transmit diverseinformation of all kinds also drives the desire to utilize aircraftsystems to send data. However, because of the large number of requiredtransponder replies in heavily travelled areas (such as in the vicinityof an airport, where hundreds of replies per second are generated),there are worldwide limits on the number of transponder broadcasttransmissions permitted each second from each aircraft. For example, thelimit for Automatic Dependent Surveillance Broadcast (ADS-B) iscurrently set to 6.2 transmissions per second to prevent the additionalADS-B interference from potentially all the aircraft near a majorairport creating a situation where the ATC ground station becomes unableto receive surveillance replies from aircraft in the terminal area beingcontrolled by ATC.

ADS-B squitter data content has already been defined for the most partby industry committees such as SC186, and there is little remaining roomfor future growth. In fact, systems currently envisioned and beingdeveloped by avionics systems designers will likely need to transmitmore data than can be sent within the 6.2 squitters per second limit.The ability to employ more data in avionic systems is now and willcontinue to be needed.

The existing Mode S transponder reply data format (also known assquitters when they are sent unsolicited by an interrogation) isimplemented with a pulse position modulation technique, where theposition of a pulse determines whether a bit is a one or a zero.Referring to the transmission reply data format and timing diagram 200in FIG. 2, the first four pulses 203 within the 8 microsecond preambletime 210 are called preamble pulses and are used to determine that thepulse position data that follows is for a Mode S reply (or squitter).ADS-B squitters use the long Mode S reply format and thus contain 112bits in the data block 220 per squitter. In other applications, 56 bitsmay be transmitted.

Data is transmitted through digital data encoded in the Data Block 220.A bit interval 202 comprises two sub-intervals defining the logicalstate of a bit symbol. When a pulse is in the “1” sub-interval position(FIG. 3, 301) of a bit interval 202, that bit value is a 1 and when apulse is in the “0” sub-interval position (FIG. 3, 302) of a bitinterval 202, that bit value is a 0. Only one pulse either in a “0” or a“1” position is permitted for each bit interval or bit symbol period(such as bit interval 202) of the entire message shown 200.

Referring to FIG. 3, an expanded view of bit interval 202 is shown. Acarrier wave in the form of a pulse waveform 310 is being transmitted inthe “1” position 301, and no pulse is transmitted 315 during the “0” subinterval, and therefore, this bit interval represents the value oflogical “1.” Note that the sinusoidal waveform 310 provided in thedrawing is for illustration purposes only, and as a standard frequencyfor ADS-B replies is currently 1090 MHz±1 MHz, approximately 545 cyclesof the waveform 310 would normally occur during the 0.5 microsecondsub-interval 301. The phase of the waveform 310 is also unimportant forexisting transponder reply standards. What is needed are methods andsystems to make more efficient use of transponder reply messages toincrease data throughput and provide for additional communication links.

SUMMARY OF THE INVENTION

Embodiments of the present invention disclose systems and methods forproviding an ATC Overlay data link. Through embodiments of the presentinvention, existing ATC (or other) modulated signals using existingstandard frequencies may be utilized to transmit (e.g., from an aircrafttransponder) additional information in a manner that does not render thetransmitted signal unrecognizable by legacy ATC equipment. Legacyequipment will be able to demodulate and decode information that wasencoded in the transmitted signal in accordance with preexistingstandard modulation formats, and updated equipment can also extract theadditional information that was overlaid on transmitted signals.

The overlay data may comprise any information desired to be transmitted,including but not limited to additional or supplementary ATCinformation. Existing transponder and ground station frequencies may beutilized for the transmission, and the signals modulated with theoverlay data may be received by any receiver, including but not limitedto receivers in aircraft and ground stations. The modulation protocolsor types utilized in both in the primary ATC signal and the overlaidsignal are selectable from any suitable modulation schemes, but those ofskill in the art appreciate that modulation protocols utilized invarious embodiments of the invention may be selected to be a non-varyingmodulation, including, but not limited to fixed modulation protocols. Asused herein, the term “overlay modulation” includes modulating a signalthat has previously been modulated, including chases where a single or aplurality of modulations were previously applied to the signal.

Embodiments provide a method for encoding an overlaid message onto aprovided modulated ATC signal, the method comprising selecting anoverlay modulation protocol; and modulating the provided modulated ATCsignal with an overlay message using the selected overlay modulationprotocol. The provided modulated ATC signal may be modulated with anyprotocol such as a pulse position modulation protocol, and the overlaymodulation protocol may be any protocol such as phase shift keyingmodulation. When phase shift keying modulation is utilized, phase statesmay be assigned using Gray Code to further reduce bit error. In oneembodiment, modulating the provided modulated ATC signal with an overlaymessage using the selected overlay modulation protocol further comprisesapplying the overlay modulation to one or more modulated message bitintervals within a data block in the modulated ATC signal. In variousembodiments where the overlay modulation comprises a PSK modulationprotocol, a phase transition in a carrier signal in a bit interval ismodulated into the carrier, and such phase transition may occur at anylocation in the carrier signal in the bit interval. In one embodiment,such phase transition may occur in the carrier signal proximate to amid-point of the bit interval, or proximate an initial point of the bitinterval. A plurality of phase transitions may also be modulated intothe carrier signal within any particular bit interval. The location of aphase transition within a bit interval may be selected to optimizespectral performance, noise performance, or any other criterion.

The ATC modulation protocol and the overlay modulation protocol maycomprise any single or combination digital modulation scheme, including,but not by way of limitation any of the following modulation protocols:binary phase shift keying modulation (BPSK); quadrature phase shiftkeying modulation (QPSK); 8-phase shift keying modulation (8-PSK);differential phase-shift keying (DPSK); DNPSK modulation, where N is aneven integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); or anyother modulation form using amplitude, phase, or frequency signalcharacteristics and methods of use; and combinations of any of thoseprotocols. Single or multiple modulation operations may be performed onthe ATC signal to encode additional overlay information. The modulatedATC signal may be a standard ATC format as described above, or maycomprise any number of signal types alone or in combination, such as atransponder reply signal; a squitter signal; an ADS-B reply signal; a1030 MHz interrogation signal; a 1030 MHz TCAS signal; a GeneralAviation UAT transceiver signal on a UHF frequency band; at least one ofa signal within a voice band or a data band operating within an ATC HF,VHF, and UHF band, a VDL Mode 4 ADS-B modulated signal; DME; SATCOM; orany other avionics equipment that radiates a modulated rf signal; abaseband signal for transmitting data; and combinations of those signaltypes.

The overlay data may be encoded, encrypted, supplemented, interleaved,or otherwise processed before or after overlay modulation occurs. Suchprocessing may be intended to reduce bit error rates by providingsupplemental check bits, parity bits, CRC bits, Reed-Solomon bit errordetection and correction coding or data, or other information to conducterror checking or error correction coding. Interleaving may be used tospread overlay data or data symbols across multiple bit intervals withina transponder reply or squitter message, or across multiple reply orsquitter messages in order to reduce the effect of burst noise on thebit error rate of the transmitted message.

In another embodiment, a method for decoding an overlaid message from anoverlay modulated signal is provided. The method includes demodulatingthe overlay modulated signal with a first modulation protocol to producea first message; demodulating the overlay modulated signal with a secondmodulation protocol to produce an overlay message; wherein the firstmessage and the overlay message may be independently demodulated fromthe overlay modulated signal.

The first or second modulation protocol may comprise any single orcombination digital modulation scheme, including, but not by way oflimitation any of the following modulation protocols: binary phase shiftkeying modulation (BPSK); quadrature phase shift keying modulation(QPSK); 8-phase shift keying modulation (8-PSK); differentialphase-shift keying (DPSK); DNPSK modulation, where N is an even integerand a multiple of 2; frequency shift keying (FSK); amplitude shiftkeying (ASK); quadrature amplitude modulation (QAM); orthogonalfrequency-division multiplexing (OFDM); minimum-shift keying (MSK);asymmetric phase-shift keying, (APSK); pulse position modulation (PPM);amplitude modulation (AM); frequency modulation (FM); or any othermodulation form using amplitude, phase, or frequency signalcharacteristics and methods of use; and combinations of any of thoseprotocols.

Embodiments of the present invention provide that the received overlaydata may be decoded, decrypted, supplemented, de-interleaved, orotherwise processed after receipt. Such processing may be intended toreduce bit error rates by providing supplemental check bits, paritybits, CRC bits, Reed-Solomon bit error detection and correction, orother information to conduct error checking or error correction coding.De-interleaving may be used to extract overlay data from multiple bitintervals within a transponder reply or squitter message, or acrossmultiple reply or squitter messages in order to reduce the effect ofburst noise on the bit error rate of the transmitted message. Further,additional demodulations may occur to extract additional overlaid datafrom the received signal. These additional demodulations may use anymodulation protocol as defined herein.

When the overlay modulation is phase shift keying-type (PSK) modulation,recovery of encoded data phases may be complicated by fluctuations ordrift in the phase of the carrier frequency. Normally, with ATCRBS orADS-B type replies, the phase of the carrier does not matter as long asthe pulse position is correctly modulated. To obtain an accurate dataphase and reduce bit errors of the PSK, embodiments of the presentinvention compensate for the phase drift whether synchronous ordifferential PSK techniques are utilized. In one embodiment, the secondmodulation protocol comprises phase shift keying; and a phase error issubtracted from a signal phase to produce a data phase. The phase errormay be computed by identifying one or more preamble pulses within theoverlay modulated signal; identifying one or more data pulses within theoverlay modulated signal; determining one or more phases of a carrierfrequency within at least one of the one or more preamble pulses and theone or more data pulses; and comparing the one or more phases of thecarrier frequency to a predetermined frequency to compute a phase error.In the case of a differential PSK modulation, embodiments of the presentinvention provide that the second modulation protocol comprisesdifferential phase shift keying; and a first phase of a first carrierwave pulse within a first bit interval is computed by comparing thefirst phase of the first carrier wave pulse within the first bitinterval to a second phase of a second carrier wave pulse within asecond bit interval.

In another embodiment, a method of the present invention provides fortransmission and receipt of overlay data through an ATC-type response orsquitter signal. Embodiments provide an overlay data link through aprovided ATC signal modulated with a first modulation protocol, througha method comprising: modulating the provided modulated ATC signal withan overlay message using a second modulation protocol to produce anoverlay modulated signal; transmitting the overlay modulated signal byan ATC transponder; receiving the overlay modulated signal at a receiversuch as with a TCAS receiver; and extracting an overlaid message from anoverlay modulated signal, wherein a first message is obtained bydemodulating the overlay modulated signal with the first modulationprotocol; and the overlay message is retrieved from the overlaymodulated signal by demodulating the overlay modulated signal with thesecond modulation protocol. As described previously, the first andoverlay modulation protocols may comprise any type of digital modulationschemes, such as one embodiment where the first modulation protocol ispulse position modulation and the overlay modulation protocol is phaseshift keying modulation. Alternatively, the first or second modulationprotocol may comprise any single or combination digital modulationscheme, including, but not by way of limitation any of the followingmodulation protocols: binary phase shift keying modulation (BPSK);quadrature phase shift keying modulation (QPSK); 8-phase shift keyingmodulation (8-PSK); differential phase-shift keying (DPSK); DNPSKmodulation, where N is an even integer and a multiple of 2; frequencyshift keying (FSK); amplitude shift keying (ASK); quadrature amplitudemodulation (QAM); orthogonal frequency-division multiplexing (OFDM);minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulseposition modulation (PPM); amplitude modulation (AM); frequencymodulation (FM); or any other modulation form using amplitude, phase, orfrequency signal characteristics and methods of use, and combinations ofany of those protocols.

There is also provided an ATC overlay data link system. One embodimentcomprises a first modulator, the first modulator configured to modulateATC data into a first modulated signal through a first modulationprotocol; a second modulator coupled to the first modulator, the secondmodulator configured to modulate an overlay message into the firstmodulated signal using a second modulation protocol to produce anoverlay modulated signal; a transponder comprising a transmitter and anantenna wherein the transponder is coupled to the second modulator andthe transponder is configured to transmit the overlay modulated signal;and a TCAS receiver coupled to an antenna, a first demodulator and asecond demodulator, wherein the first demodulator is configured toextract the ATC data by demodulating the overlay modulated signal withthe first modulation protocol; and the second demodulator is configuredto extract the overlaid message from an overlay modulated signal.

As described previously, the first and overlay modulation protocols maycomprise any type of digital modulation schemes, such as one embodimentwhere the first modulation protocol is pulse position modulation and theoverlay modulation protocol is phase shift keying modulation.Alternatively, the first or second modulation protocol may comprise anysingle or combination digital modulation scheme, including, but not byway of limitation any of the following modulation protocols: binaryphase shift keying modulation (BPSK); quadrature phase shift keyingmodulation (QPSK); 8-phase shift keying modulation (8-PSK); differentialphase-shift keying (DPSK); DNPSK modulation, where N is an even integerand a multiple of 2; frequency shift keying (FSK); amplitude shiftkeying (ASK); quadrature amplitude modulation (QAM); orthogonalfrequency-division multiplexing (OFDM); minimum-shift keying (MSK);asymmetric phase-shift keying, (APSK); pulse position modulation (PPM);amplitude modulation (AM); frequency modulation (FM); or any othermodulation form using amplitude, phase, or frequency signalcharacteristics and methods of use; and combinations of any of thoseprotocols.

Embodiments of the present invention provide that the system mayencode/decode, encrypt/decrypt, supplement, interleave/de-interleave, orotherwise process data for overlay and data extraction. Such processingmay be intended to reduce bit error rates by providing supplementalcheck bits, parity bits, CRC bits, Reed-Solomon Bit Error Detection andCorrection, or other information to conduct error checking or errorcorrection coding. Interleaving/De-interleaving may be used to extractoverlay data from multiple bit intervals within a transponder reply orsquitter message, or across multiple reply or squitter messages in orderto reduce the effect of burst noise on the bit error rate of thetransmitted message. Further, additional demodulations may occur toextract additional overlaid data from the received signal. Theseadditional demodulations may use any modulation protocol as definedherein.

There is further provided an overlay data link embodiment. The overlaydata link comprises a first modulator, the first modulator configured tomodulate a first data stream data into a first modulated signal througha first modulation protocol; a second modulator coupled to the firstmodulator, the second modulator configured to modulate an overlaymessage into the first modulated signal using a second modulationprotocol to produce an overlay modulated signal; a transmitter, whereinthe transmitters is coupled to the second modulator and the transmitteris configured to transmit the overlay modulated signal; and a receiver,a first demodulator and a second demodulator, wherein: the firstdemodulator is configured to extract first data stream by demodulatingthe overlay modulated signal with the first modulation protocol; and thesecond demodulator is configured to extract the overlaid message from anoverlay modulated signal. In one embodiment of the overlay data link,the first modulation protocol is pulse position modulation and theoverlay modulation protocol is phase shift keying modulation. The firstand second modulation protocols may comprise any appropriate amplitude,phase, and frequency modulation, including, but not limited to binaryphase shift keying modulation (BPSK); quadrature phase shift keyingmodulation (QPSK); 8-phase shift keying modulation (8-PSK); differentialphase-shift keying (DPSK); DNPSK modulation, where N is an even integerand a multiple of 2; frequency shift keying (FSK); amplitude shiftkeying (ASK); quadrature amplitude modulation (QAM); orthogonalfrequency-division multiplexing (OFDM); minimum-shift keying (MSK);asymmetric phase-shift keying, (APSK); pulse position modulation (PPM);amplitude modulation (AM); frequency modulation (FM); and combinationsthereof.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an embodiment of a system of thepresent invention.

FIG. 2 illustrates a standard prior art transponder reply data formatand timing diagram.

FIG. 3 depicts an expanded view of an exemplary bit interval of thereply data format, with a logical data value “1” pulse encoded.

FIG. 4 shows one bit interval of modulated ATC signal with overlay data,where the carrier has been phase shifted ninety degrees, the phase shiftoccurring at or near the beginning of the bit interval.

FIG. 5 shows one bit interval of modulated ATC signal with overlay data,where the carrier has been phase one hundred eighty degrees, the phaseshift occurring approximately at the midpoint of a carrier within asub-bit interval.

FIG. 6 shows one bit interval of modulated ATC signal with overlay data,where the carrier has been phase shifted ninety degrees, and frequencydrift has affected the carrier.

FIG. 7 illustrates one and one half bit intervals, illustrating a mergedcarrier signal from a bit value of 0 from a previous bit interval, and abit value of 1 from a following bit interval with midpoint one hundredeighty degree phase shifts.

FIG. 8 shows an exemplary modulator of the present invention.

FIG. 9 illustrates an exemplary demodulator of the present invention.

FIG. 10 shows a constellation diagram and corresponding Gray Code symbolassignments for an 8-PSK modulation scheme of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present exemplaryembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Embodiments of the present invention provide methods for overlayingadditional data on top of existing ATC data without affecting existingATC transponder/TCAS/Ground Station/Multilateration or other existingequipment. Embodiments of the present invention may use any form ofmodulation overlaid on top of existing ATC data, and depending onimplementation, the overlaid data may be undetectable by equipment thatprocesses legacy ATC signals. This permits the use of existing equipmentwithin the ATC environment, and the use of new equipment that can beeither backwards compatible and/or independent of ATC existing dataformats permitting more data throughput efficiency using the same ATCbandwidth and frequency assignments of 1030 and 1090 Mhz. Embodiments ofthe present invention utilize a transponder reply data format and timingdiagram similar to that shown in FIG. 2. However, as discussed in moredepth below, embodiments of the present invention provide additionalmodulation within each pulse interval 202 that may not be detected byexisting ATC equipment or methods, but can provide additional data bitswithin the same reply.

Turning to FIG. 1, a block diagram 100 illustrates an embodiment of asystem of the present invention. A primary ATC data stream 103 is inputto an encoder and modulator 105 that produces a modulated signal (suchas by pulse position modulation) that complies with existing standardssuch as an ADS-B squitter or reply transmissions (see, e.g., FIG. 2).Data 104 that is desired to be transmitted through an overlay data linkis encoded 110, if desired, with information such as parity bits, CRC(cyclic redundancy check) codes, encryption keys, or any otherinformation or encodings that are desired to be applied to the overlaydata. The encoded overlay data is then further 115 modulated into themodulated ATC stream 115 in a manner that provides a reply/squittersignal that is compatible with existing hardware yet containsinformation in excess of that defined by current transponder standards.One exemplary method to achieve such overlay modulation compatibility isto initially modulate the ATC signal with a pulse position modulationthat is acceptable by currently deployed hardware, and then apply theoverlay modulation with a technique such as phase shift keying “PSK.”PSK is a digital modulation scheme that conveys data by changing, ormodulating, the phase of a reference signal (e.g., the carrier wave), asopposed to selectively applying a pulse to bit symbol times as isutilized in pulse position modulation. Since varying the phase of thecarrier signal within each defined time frame in an ATC-encoded replysignal does not impact the ability of legacy hardware to decode theoriginal ATC data message, the PSK modulation is nondestructive and maybe independently demodulated. Put another way, the twice-modulatedsignal 117 carries the ATC data 103 that is modulated and directlydecodable by conventional PPM techniques, in addition to additional data104 that has been overlaid on the modulated signal in a non-destructivemanner, such as by PSK modulation.

The modulated signal 117 may then be transmitted 120 and received by areceiver 125 that is configured to receive ATC transponder replytransmissions (such as a ground station or another transponder in asuitably equipped aircraft). In various embodiments, the transmitter 120and the receiver 125 may comprise any electronic equipment capable ofsending and/or receiving RF signals, including, but not limited to ATCradars, TCAS transponders, ADS-B transponders, and ground stations ofany type. In one embodiment, ADS-B ground stations can receive andtransmit ATC Overlay messages on top of various outputs such as TrafficInformation Service Broadcasts (TIS-B, Automatic Dependent SurveillanceRe Broadcasts (ADS-R).

The received signal is then demodulated 135 by PPM demodulation 135 torecover the original primary ATC data 103 after any necessary decoding.The received signal is also demodulated 130 to obtain the overlay data104, which may be presented in several channels after any necessarydecoding. Therefore, the twice-modulated signal 117 may be demodulatedwith multiple techniques independently and each data stream (103, 104)respectively independently recovered.

Any form of modulation may be overlaid on top of any ATC-modulatedsignals, whether such modulation currently exists or is created at afuture date. In alternate embodiments, additional data may be overlaidby modulation on top of an overlaid modulation as well, and thismodulation is not limited to modulation that is undetectable by existingequipment. Further, the modulation is not limited to 1090 MHz squittersand replies, but can also be used on 1030 MHz (currently interrogationby radars and TCAS), on the General Aviation UAT transceiver UHFfrequency band, ATC HF, VHF and UHF voice and data bands, on the VDLMode 4 ADS-B modulation used for ADS-B in limited locales within Europe,DME; SATCOM; or any other avionics equipment that radiates a modulatedRF signal and at baseband for ATC terminal or other networks currentlysending data over landline or microwave links. Thus any form of ATCmodulation that is used can have an overlaid modulation applied to it atany frequency. This can also include, for example, overlaid modulationon a modulation applied to Air Transport airborne weather radar forcommunication links. Therefore, embodiments of the present inventionprovide that any overlay modulation type (whether interfering in somemanner or not) may be overlaid on or added to or encrypted with anymodulation type for whatever purpose desired, such as, but not limitedto ATC applications, any commercial data relay purposes, low probabilityof intercept applications, low probability of detection techniques, lowprobability of spoofing purposes, and encryption.

Embodiments of the present invention may employ phase shift keying (PSK)modulation within each of the 112 bit pulses in the data block 220.Turning to FIG. 4, an expanded view of one exemplary bit interval 202 ofthe data block 220 is shown, with an ATC data value of logical “1”(carrier pulse 400 is present in “1” sub-interval position, 301) and thephase shifted carrier signal 400 is overlaid with phase modulation byphase shift 410 of ninety degrees. The phase shift in this embodimentoccurred at or near the beginning 403 of the logical “1” sub-intervalposition 301. The original unshifted carrier signal 310 is shown forcomparison. Depending on the particular PSK implementation, this phaseshift may correspond to one bit or several bits of overlay data. Notethat the sinusoidal waveform 310 provided in the drawing is forillustration purposes only, and as a standard frequency for ADS-Breplies is currently 1090 MHz±1 MHz, approximately 545 cycles of thewaveform 310 would normally occur during the 0.5 microsecondsub-interval 301. As the applicable ATC standards do not care about thephase of the carrier wave, the presence of an acceptable frequency andamplitude waveform in the appropriate bit sub-interval position 301 isall that is needed to provide the logical “1” originally encoded.

Phase information can then be sent with a differential phase betweeneach pulse representative of a plurality of states used to represent aplurality of bits. Each phase difference is detected by examining thephase of a previous pulse to the phase of the next pulse to eliminatethe effects of relative aircraft motion. For the first phase referencepulse relative to Bit 1 case, any of the preamble pulses 203 may beused, and from then forward each previous bit pulse phase can be used asthe reference for the next bit phase.

Other PSK methods can be used to send data such as a synchronoustransmission of phase, where comparison to a reference that issynchronized in phase to the incoming reply signal modulation is used todetermine a phase value for each pulse. The preamble pulses of the ModeS reply message can be used to synchronize a reference oscillator sourceto the incoming message RF signal using, for instance, aphase-locked-loop oscillator or a Costas loop that can be locked to theincoming signal phase and frequency, which is then used as a phasereference for all the data bits.

In legacy ATC systems, the frequency of the signal being transmitted arein the range of 1090 MHz plus or minus 1 MHz, and drift of the phase ofthe carrier signal is generally not of concern provided the signalenvelopes within predetermined timing intervals is within establishedranges, such as those set forth in RTCA standard DO-181C. To recover aphase shift encoded within the broadcast carrier signal, embodiments ofthe invention provide for obtaining the data phase by synchronous orasynchronous means. Turning to FIG. 6, original carrier 310 (shown onlypartially for clarity) would have undergone a frequency drift 610,producing a new drifted-phase carrier 612. Without knowledge of theamount of drift 610, the exact phase of the phase-shifted carrier 400may be difficult to determine. In one embodiment, phase drift 610 isaccounted for via phase synchronous means but with the frequencyasynchronous, for instance by utilizing an integrator and accumulatingand storing the phase drift as a phase error signal that can besubtracted out from the phase of the current signal. In anotherillustrative embodiment, after a predetermined amount of data pulses arereceived (for instance 7 pulses), a local oscillator tracks the receivedsignal via a Costas loop, allowing frequency and phase synchronization.The local oscillator is then driven with an offset frequency to maintainsynchronization. Through this approach, interfering signals withdifferent frequencies can be distinguished through use of a matchedfilter, and through use of matched filters with synchronized frequencyand phase, noise performance can be improved significantly, for instanceby about 3 dB.

Aircraft motion and signal-to-noise-ratio, as mentioned above, may betaken into account when determining an acceptable bit error rate (BER).For instance, if two aircraft, one transmitting a reply and the onereceiving the reply are traveling towards one another at a rate of 1200knots, which is 2000 ft. per second, the relative velocities cansignificantly affect the phase error seen by the receiving aircraft.Since the wavelength in free space is about 1 foot at ATC 1030 MHz and1090 MHz frequencies, a phase error of about 2000 ft./sec.×360 degreesphase×112 usec (112×10⁻⁶ seconds per Mode S reply message length)=80degrees phase error. Since the 80 degrees of error is spread over theentire Mode S message and each phase can either be differentiallycompared from the previous to the next bit or synchronized out asdescribed above, the phase error between each bit then is about 80degrees/112 bits per microsecond=0.71 degrees phase error per bit. Thus,any encoding scheme that can tolerate a phase error of 0.71 degreesbetween each bit is realizable, but is also affected by the signal tonoise ratio to accurately measure phase. For a reasonable signal tonoise ratio that exceeds 10 db, a PSK encoding scheme that provides 360states or 1 degree per state could be tolerated. Usually a power of twois used for binary encoding, so 256 (2⁸) states could be used to provide8 bits of data per PPM pulse.

To decrease the effects of noise on signal to noise ratio, embodimentsof the present invention use a D8PSK (Differential 8-state Phase ShiftKeying) modulation scheme. Referring to FIG. 10, each of 8 states isshown from 0 degrees through 360 degrees with each state separated by 45degrees representing 3 bits. This provides 112 bits×3=336 bits permessage additional to the 112 bits of the original Pulse PositionModulation (PPM) Mode S reply message. Thus, if an additional 3 bitmessage is sent for each of 6.2 squitters per second, a total of 336bits per message×6.2 messages=2083 additional bits can be sent via theoverlay modulation. Therefore, by modulating each pulse-bit whether inthe zero or one position, with D8PSK modulation, a new data link is thencreated with 3 bits of data for every previous PPM bit. In oneembodiment, the new data link would not be detected by existing ATC TCASand transponder equipment unless it is equipped to detect the D8PSKmodulation, ensuring backward compatibility with existing systems.

In various embodiments, states can be encoded to reduce the number ofbit errors per symbol. Other schemes using additional parity bits toproduce symbols that are completely orthogonal can be used to provideadditional interference immunity.

In alternate embodiments, modulation schemes providing more bits persecond such as D16PSK (4 bits per message bit) or D32PSK (5 bits permessage bit) can be used depending on the amount of noise immunityversus data rate required. Secondary modulation schemes may also be usedin conjunction with error correction and control schemes in order tominimize bit error rate and correspondingly increase signal to noiseratios in noisy environments. Also, bits can be encoded into states thatonly permit one bit change per adjacent state change (Grey Code). Thiscan reduce bit errors to only one bit for changes between adjacentstates and helps with noise and interference immunity, as discussedlater.

Additional techniques can be used to minimize frequency spectral powereffects or bandwidth required to accommodate the additional modulation.For instance, when two pulses are positioned next to one another, suchas shown in FIG. 7, and in the case of a sudden change in phase 720 inthe first pulse 705 to a different phase in the next pulse 710, anamplitude notch may occur between pulses affecting the bandwidth of thereply. However, if the phase is slowly varied between the middle of thefirst pulse to the middle of the second pulse, the phase variationoccurs over a greater time period and the amplitude notch can be made tobe very small, permitting compliance to existing bandwidth requirementsfor Mode S replies and having no effects upon existing equipment in thefield. The phase information can then be read in the beginning of thepulse for the first bit interval (before any phase changes to the nextpulse) and near the end of the second pulse for the second bit interval(after a phase change from the first to the second pulse is complete)when two pulses have merged into one, permitting the use of this MSK(Minimum Shift Keying) phase change technique. This approach also can beadapted for significant or multiple intra-sub-interval phase shifts, ifembodiments provide for multiple phase shifts per sub-interval.

MSK type of modulation phase change across the pulse will further reduceany spectral effects. The worst case spectral effect is for a phaseshift from 0 to 180 degrees, and could be accommodated and tested toensure compliance with existing industry specifications for a Mode Sreply. Spectral and amplitude specifications for Mode S reply emissionscan currently be found in RTCA DO-181C.

Additional modulation types with varying degrees of BER, detection gain,and bit throughput can be used. These include, but are not limited to:general phase shift keying modulations; binary phase shift keyingmodulation (BPSK); quadrature phase shift keying modulation (QPSK);8-phase shift keying modulation (8-PSK); differential phase-shift keying(DPSK); DNPSK modulation, where N is an even integer and a multiple of2; frequency shift keying (FSK); amplitude shift keying (ASK);quadrature amplitude modulation (QAM); orthogonal frequency-divisionmultiplexing (OFDM); minimum-shift keying (MSK); asymmetric phase-shiftkeying, (APSK); pulse position modulation (PPM); amplitude modulation(AM); frequency modulation (FM); or any other modulation form usingamplitude, phase, or frequency signal characteristics and methods ofuse; and combinations of modulation techniques.

In principle, any modulation type can be used, but should be constrainedto meet DO-181C amplitude and spectral requirements such that existingATC equipment is not affected, but new equipment can transmit and detectthe overlaying modulation. In addition, some form of bit error detectionand correction can be used such as that presented in RTCA DO-181C, ModeS Minimum Operational Performance Specification, and RTCA DO-185A, TCASMinimum Operational Performance Specification, as an example to improvemessage reception.

A more suitable means of bit error detection and correction for PhaseShift Keying modulation can, for example, include the use of additionalbits to provide orthogonal symbol (cross correlation is zero for one biterrors) encoding with zero correlation between symbol sets (bit patternsbetween symbols are significantly different by several bit states). Forexample, the bit values can be represented, as shown below in Table 1(for two bits to simplify the discussion). Note that this method mayrequire that more bits be used within each symbol to represent thelesser number of bit values (or states), so that a more unique bitpattern is transmitted and received, and can be corrected in thepresence of noise or interference with a higher degree of certainty.

TABLE 1 Bit Values Symbol Data 00 0000 01 0101 10 1100 11 1001

Another embodiment of the present invention addresses limiting theeffect of burst errors, such as for a single interfering pulse where the“BIT 1” pulse position of the Mode S reply has been overlaid by astronger in amplitude interfering pulse. This situation could destroy anentire 3 bit symbol for the case of D8PSK (3 bits per symbol). However,if each D8PSK bit value is sent as only one bit of 3 separate messages,then only one bit of each message may be affected, making it possiblefor 2 of the messages to remain uncorrupted. For example, from Table 1,the last bit of the Symbol Data for Bit Value 10 could be corrupted intoa 1, resulting in the sequence from Table 1 to be a 1101 instead of a1100. Then, the 1101 is incorrect, since it is not a valid symbol and byknowing which bit has been corrupted from the interfering pulse positionit is possible to determine that 1100 is the correct symbol for a onebit error, since none of the other symbols are of the form “11XX”.

Non-ideal communications channels, noise, atmospheric conditions,interference, or other phenomena may induce errors into the messagebeing communicated, whether or not the transmitted message has beenoverlaid with additional encoded information. In various embodiments ofthe present invention, conventional error reduction or correctionapproaches may be applied to reduce bit error rate, such as utilizationof a parity overlay or CRC check scheme as described in RTCA/DO-181C,Reed-Solomon bit error detection and correction, or other errorconventional detection and correction schemes.

Embodiments of the present invention include the use of a Gray-Code foreach phase state transmitted which is most useful for the case where aninterfering pulse is near to the same amplitude as the data pulse andcannot be detected, use of a four bit message symbol where only oneparity bit is used for every three message bits to detect and correctsingle bit errors (as opposed to traditional 8 bit symbols where halfthe bits are parity bits), interleaving of message bit symbols to limitthe number of errors per message symbol due to sequential interferingpulses so that most errors can be corrected (after de-interleaving) bychanging the state of a single bit, and using a parity overlay encodingscheme on top of the last 24 bits of the address of the intendedrecipient for point to point messaging or with an all ones address forbroadcast messages as is done now for Automatic Dependent Broadcastmessages (ADS-B) as described in RTCA DO-260A ADS-B MOPS in conjunctionwith the error detection and correction algorithm as described in RTCADO-185A TCAS MOPS. Alternate embodiments provide methods for bit errordetection and correction, and may be more efficient in terms of thenumber of phase message bits that can be corrected per each 112 bit PPMMode S reply.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A method for encoding an overlaid message onto a provided modulatedATC signal, the method comprising: selecting an overlay modulationprotocol; and modulating the provided modulated ATC signal with anoverlay message using the selected overlay modulation protocol.
 2. Themethod as disclosed in claim 1, wherein the provided modulated ATCsignal is modulated with pulse position modulation protocol.
 3. Themethod as disclosed in claim 1, wherein the overlay modulation protocolis phase shift keying modulation.
 4. The method as disclosed in claim 3,wherein a plurality of phase states for sequentially defined phases areGray-Code encoded.
 5. The method as disclosed in claim 1, whereinmodulating the provided modulated ATC signal with an overlay messageusing the selected overlay modulation protocol further comprisesapplying the overlay modulation to a carrier signal within one or moremodulated message bit intervals within a data block in the modulated ATCsignal.
 6. The method as disclosed in claim 5, wherein: the overlaymodulation a PSK protocol; and a phase transition is modulated into thecarrier signal proximate to a mid-point of the bit interval.
 7. Themethod as disclosed in claim 5, wherein: the overlay modulation a PSKprotocol; and a phase transition is modulated into the carrier signalproximate an initial point of the bit interval.
 8. The method asdisclosed in claim 5, wherein: the overlay modulation a PSK protocol;and a plurality of phase transitions are modulated into the carriersignal within one bit interval.
 9. The method as disclosed in claim 5,further comprising transmitting the overlay-modulated ATC signal from atleast one of a TCAS transponder; an ADS-B transponder; a ATC radar; anda ground station.
 10. The method as disclosed in claim 1, wherein theoverlay modulation protocol is selected from the group consisting of:binary phase shift keying modulation (BPSK); quadrature phase shiftkeying modulation (QPSK); 8-phase shift keying modulation (8-PSK);differential phase-shift keying (DPSK); DNPSK modulation, where N is aneven integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); andcombinations thereof.
 11. The method as disclosed in claim 1 wherein theoverlay modulation protocol is selected from the group consisting of:modulation by amplitude characteristics, modulation by phasecharacteristics, modulation by frequency characteristics, and anycombination thereof.
 12. The method as disclosed in claim 1, wherein theprovided modulated ATC signal comprises a signal selected from the groupconsisting of: a transponder reply signal; a squitter signal; an ADS-Breply signal; a 1030 MHz interrogation signal; a 1030 MHz TCAS signal;by a General Aviation UAT transceiver signal on a UHF frequency band; atleast one of a signal within a voice band or a data band operatingwithin an ATC HF, VHF, and UHF band, a VDL Mode 4 ADS-B modulatedsignal; a DME signal; SATCOM signal; a signal originating from anyavionics equipment that radiates a modulated RF signal; a basebandsignal for transmitting data; and combinations thereof.
 13. The methodas disclosed in claim 1 further comprising encrypting the overlaymessage.
 14. The method as disclosed in claim 1 further comprisingapplying a second overlay modulation protocol to the overlay-modulatedsignal.
 15. The method as disclosed in claim 14, wherein the secondoverlay modulation protocol is selected from the group consisting of:binary phase shift keying modulation (BPSK); quadrature phase shiftkeying modulation (QPSK); 8-phase shift keying modulation (8-PSK);differential phase-shift keying (DPSK); DNPSK modulation, where N is aneven integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); andcombinations thereof.
 16. The method as disclosed in claim 1 wherein thesecond overlay modulation protocol is selected from the group consistingof: modulation by amplitude characteristics, modulation by phasecharacteristics, modulation by frequency characteristics, and anycombination thereof.
 17. The method as disclosed in claim 1 furthercomprising encoding, within the overlay message, at least one of: one ormore parity bits; one or more CRC bits; Reed-Solomon bit error detectionand correction data; and one or more error correction code bits.
 18. Themethod as disclosed in claim 1 wherein modulating the provided modulatedATC signal with an overlay message using the selected overlay modulationprotocol further comprises interleaving the overlay message into messagebit intervals.
 19. A method for decoding an overlaid message from anoverlay modulated signal, the method comprising: demodulating theoverlay modulated signal with a first modulation protocol to produce afirst message; demodulating the overlay modulated signal with a secondmodulation protocol to produce an overlay message; and wherein the firstmessage and the overlay message may be independently demodulated fromthe overlay modulated signal.
 20. The method as disclosed in claim 19,further comprising receiving the overlay modulated signal in at leastone of a TCAS transponder; an ADS-B transponder; an ATC radar; and aground station.
 21. The method as disclosed in claim 19, wherein thefirst modulation protocol is selected from the group consisting of:binary phase shift keying modulation (BPSK); quadrature phase shiftkeying modulation (QPSK); 8-phase shift keying modulation (8-PSK);differential phase-shift keying (DPSK); DNPSK modulation, where N is aneven integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); andcombinations thereof.
 22. The method as disclosed in claim 19, whereinthe first modulation protocol is selected from the group consisting of:modulation by amplitude characteristics, modulation by phasecharacteristics, modulation by frequency characteristics, and anycombination thereof.
 23. The method as disclosed in claim 19, whereinthe second modulation protocol is selected from the group consisting of:binary phase shift keying modulation (BPSK); quadrature phase shiftkeying modulation (QPSK); 8-phase shift keying modulation (8-PSK);differential phase-shift keying (DPSK); DNPSK modulation, where N is aneven integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); andcombinations thereof.
 24. The method as disclosed in claim 19, whereinthe second modulation protocol is selected from the group consisting of:modulation by amplitude characteristics, modulation by phasecharacteristics, modulation by frequency characteristics, and anycombination thereof.
 25. The method as disclosed in claim 19 furthercomprising decrypting the overlay message.
 26. The method as disclosedin claim 19 further comprising demodulating the overlay modulated signalwith a third modulation protocol, producing a second overlay message.27. The method as disclosed in claim 26, wherein the third overlaymodulation protocol is selected from the group consisting of: binaryphase shift keying modulation (BPSK); quadrature phase shift keyingmodulation (QPSK); 8-phase shift keying modulation (8-PSK); differentialphase-shift keying (DPSK); DNPSK modulation, where N is an even integerand a multiple of 2; frequency shift keying (FSK); amplitude shiftkeying (ASK); quadrature amplitude modulation (QAM); orthogonalfrequency-division multiplexing (OFDM); minimum-shift keying (MSK);asymmetric phase-shift keying, (APSK); pulse position modulation (PPM);amplitude modulation (AM); frequency modulation (FM); or any othermodulation form using amplitude, phase, or frequency signalcharacteristics and methods of use; and combinations thereof.
 28. Themethod as disclosed in claim 26, wherein the third overlay modulationprotocol is selected from the group consisting of: modulation byamplitude characteristics, modulation by phase characteristics,modulation by frequency characteristics, and any combination thereof.29. The method as disclosed in claim 19 further comprising decoding theoverlay message using at least one of: one or more parity bits; one ormore CRC bits; Reed-Solomon bit error detection and correction data; andone or more error correction code bits.
 30. The method as disclosed inclaim 19 further comprising decoding the overlay message wherein theoverlay message is recovered from data interleaved into message bitintervals within the overlay modulated signal.
 31. The method asdisclosed in claim 19, wherein: the second modulation protocol comprisesphase shift keying; and a phase error is subtracted from a signal phaseto produce a data phase.
 32. The method as disclosed in claim 31,wherein the phase error is computed by identifying one or more preamblepulses within the overlay modulated signal; identifying one or more datapulses within the overlay modulated signal; determining one or morephases of a carrier frequency within at least one of the one or morepreamble pulses and the one or more data pulses; and comparing the oneor more phases of the carrier frequency to a predetermined frequency tocompute a phase error.
 33. The method as disclosed in claim 19, wherein:the second modulation protocol comprises differential phase shiftkeying; and a first phase of a first carrier wave pulse within a firstbit interval is computed by comparing the first phase of the firstcarrier wave pulse within the first bit interval to a second phase of asecond carrier wave pulse within a second bit interval.
 34. A method forproviding an overlay data link through a provided ATC signal modulatedwith a first modulation protocol, the method comprising: modulating theprovided modulated ATC signal with an overlay message using a secondmodulation protocol to produce an overlay modulated signal; transmittingthe overlay modulated signal by an ATC transponder; receiving theoverlay modulated signal at a receiver; and extracting an overlaidmessage from an overlay modulated signal, wherein: a first message isobtained by demodulating the overlay modulated signal with the firstmodulation protocol; and the overlay message is retrieved from theoverlay modulated signal by demodulating the overlay modulated signalwith the second modulation protocol.
 35. The method as disclosed inclaim 34, wherein the first modulation protocol is pulse positionmodulation and the overlay modulation protocol is phase shift keyingmodulation.
 36. The method as disclosed in claim 34, wherein modulatingthe modulated ATC signal with an overlay message further comprisesapplying the overlay modulation to a carrier signal within one or moremodulated message bit intervals within a data block in the modulated ATCsignal.
 37. The method as disclosed in claim 36, wherein: the overlaymodulation a PSK protocol; and a phase transition is modulated into thecarrier signal proximate to a mid-point of the bit interval.
 38. Themethod as disclosed in claim 36, wherein: the overlay modulation a PSKprotocol; and a phase transition is modulated into the carrier signalproximate an initial point of the bit interval.
 39. The method asdisclosed in claim 36, wherein: the overlay modulation a PSK protocol;and a plurality of phase transitions are modulated into the carriersignal within one bit interval.
 40. The method as disclosed in claim 34,wherein the ATC transponder is selected from the group consisting of: anATCRBS-compatible transponder; an ADS-B transponder; an ATC radar; and aground station.
 41. The method as disclosed in claim 34, wherein thefirst modulation protocol is selected from the group consisting of:binary phase shift keying modulation (BPSK); quadrature phase shiftkeying modulation (QPSK); 8-phase shift keying modulation (8-PSK);differential phase-shift keying (DPSK); DNPSK modulation, where N is aneven integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); andcombinations thereof.
 42. The method as disclosed in claim 34, whereinthe first modulation protocol is selected from the group consisting of:modulation by amplitude characteristics, modulation by phasecharacteristics, modulation by frequency characteristics, andcombinations thereof.
 43. The method as disclosed in claim 34, whereinthe second modulation protocol is selected from the group consisting of:binary phase shift keying modulation (BPSK); quadrature phase shiftkeying modulation (QPSK); 8-phase shift keying modulation (8-PSK);differential phase-shift keying (DPSK); DNPSK modulation, where N is aneven integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); andcombinations thereof.
 44. The method as disclosed in claim 34, whereinthe second modulation protocol is selected from the group consisting of:modulation by amplitude characteristics, modulation by phasecharacteristics, modulation by frequency characteristics, andcombinations thereof.
 45. The method as disclosed in claim 34 furthercomprising decoding the overlay message using at least one of: one ormore parity bits; one or more CRC bits; Reed-Solomon bit error detectionand correction data; and one or more error correction code bits.
 46. Themethod as disclosed in claim 34 further comprising decoding the overlaymessage wherein the overlay message is recovered from data interleavedinto message bit intervals within the overlay modulated signal.
 47. Themethod as disclosed in claim 34, wherein: the second modulation protocolcomprises phase shift keying; and a phase error is subtracted from asignal phase to produce a data phase.
 48. The method as disclosed inclaim 34, wherein the phase error is computed by identifying one or morepreamble pulses within the overlay modulated signal; identifying one ormore data pulses within the overlay modulated signal; determining one ormore phases of a carrier frequency within at least one of the one ormore preamble pulses and the one or more data pulses; and comparing theone or more phases of the carrier frequency to a predetermined frequencyto compute a phase error.
 49. The method as disclosed in claim 34,wherein: the second modulation protocol comprises differential phaseshift keying; and a first phase of a first carrier wave pulse within afirst bit interval is computed by comparing the first phase of the firstcarrier wave pulse within the first bit interval to a second phase of asecond carrier wave pulse within a second bit interval.
 50. An ATCoverlay data link system comprising: a first modulator, the firstmodulator configured to modulate ATC data into a first modulated signalthrough a first modulation protocol; a second modulator coupled to thefirst modulator, the second modulator configured to modulate an overlaymessage into the first modulated signal using a second modulationprotocol to produce an overlay modulated signal; a transpondercomprising a transmitter and an antenna wherein the transponder iscoupled to the second modulator and the transponder is configured totransmit the overlay modulated signal; and a receiver coupled to anantenna, a first demodulator and a second demodulator, wherein: thefirst demodulator is configured to extract the ATC data by demodulatingthe overlay modulated signal with the first modulation protocol; and thesecond demodulator is configured to extract the overlaid message from anoverlay modulated signal.
 51. The ATC overlay data link system asdisclosed in claim 50, wherein the first modulation protocol is pulseposition modulation and the overlay modulation protocol is phase shiftkeying modulation.
 52. The ATC overlay data link system as disclosed inclaim 50, wherein the first modulation protocol is selected from thegroup consisting of: binary phase shift keying modulation (BPSK);quadrature phase shift keying modulation (QPSK); 8-phase shift keyingmodulation (8-PSK); differential phase-shift keying (DPSK); DNPSKmodulation, where N is an even integer and a multiple of 2; frequencyshift keying (FSK); amplitude shift keying (ASK); quadrature amplitudemodulation (QAM); orthogonal frequency-division multiplexing (OFDM);minimum-shift keying (MSK); asymmetric phase-shift keying, (APSK); pulseposition modulation (PPM); amplitude modulation (AM); frequencymodulation (FM); and combinations thereof.
 53. The ATC overlay data linksystem as disclosed in claim 50, wherein the first modulation protocolis selected from the group consisting of: modulation by amplitudecharacteristics, modulation by phase characteristics, modulation byfrequency characteristics, and combinations thereof.
 54. The ATC overlaydata link system as disclosed in claim 50, wherein the second modulationprotocol is selected from the group consisting of: binary phase shiftkeying modulation (BPSK); quadrature phase shift keying modulation(QPSK); 8-phase shift keying modulation (8-PSK); differentialphase-shift keying (DPSK); DNPSK modulation, where N is an even integerand a multiple of 2; frequency shift keying (FSK); amplitude shiftkeying (ASK); quadrature amplitude modulation (QAM); orthogonalfrequency-division multiplexing (OFDM); minimum-shift keying (MSK);asymmetric phase-shift keying, (APSK); pulse position modulation (PPM);amplitude modulation (AM); frequency modulation (FM); and combinationsthereof.
 55. The ATC overlay data link system as disclosed in claim 50,wherein the second modulation protocol is selected from the groupconsisting of: modulation by amplitude characteristics, modulation byphase characteristics, modulation by frequency characteristics, andcombinations thereof.
 56. The ATC overlay data link system as disclosedin claim 50, wherein the second modulator is further configured toencode the overlay message using at least one of: one or more paritybits; one or more CRC bits; Reed-Solomon bit error detection andcorrection data; and one or more error correction code bits.
 57. The ATCoverlay data link system as disclosed in claim 50, wherein the receiveris further configured to decode the overlay message using at least oneof: one or more parity bits; one or more CRC bits; Reed-Solomon biterror detection and correction data; and one or more error correctioncode bits.
 58. The ATC overlay data link system as disclosed in claim 50wherein the receiver is further configured to decode the overlaymessage, wherein the overlay message is recovered from data interleavedinto message bit intervals within the overlay modulated signal.
 59. TheATC overlay data link system as disclosed in claim 50, wherein: thesecond modulation protocol comprises phase shift keying; and the seconddemodulator is configured to subtract a phase error from a signal phaseto produce a data phase.
 60. The ATC overlay data link system asdisclosed in claim 50, wherein the second demodulator is configured tocompute a phase error by: identifying one or more preamble pulses withinthe overlay modulated signal; identifying one or more data pulses withinthe overlay modulated signal; determining one or more phases of acarrier frequency within at least one of the one or more preamble pulsesand the one or more data pulses; and comparing the one or more phases ofthe carrier frequency to a predetermined frequency to compute a phaseerror.
 61. The ATC overlay data link system as disclosed in claim 50,wherein: the second modulation protocol comprises differential phaseshift keying; and the second demodulator is configured to compute afirst phase of a first carrier wave pulse within a first bit interval bycomparing the first phase of the first carrier wave pulse within thefirst bit interval to a second phase of a second carrier wave pulsewithin a second bit interval.
 62. The ATC overlay data link system asdisclosed in claim 50, wherein the first modulated signal comprises asignal selected from the group consisting of: a transponder replysignal; a squitter signal; an ADS-B reply signal; a 1030 MHzinterrogation signal; a 1030 MHz TCAS signal; by a General Aviation UATtransceiver signal on a UHF frequency band; at least one of a signalwithin a voice band or a data band operating within an ATC HF, VHF, andUHF band, a VDL Mode 4 ADS-B modulated signal; a DME signal; a SATCOMsignal; a signal originating from any avionics equipment that radiates amodulated RF signal; a baseband signal for transmitting data; andcombinations thereof.
 63. The ATC overlay data link system as disclosedin claim 50, wherein the second modulator is configured to apply theoverlay modulation to one or more modulated message bit intervals withina data block in the first modulated signal.
 64. The ATC overlay datalink system as disclosed in claim 50, wherein the receiver comprises areceiver type selected from the group consisting of: a TCAS receiver; anADS-B receiver; and a ground station receiver.
 65. The ATC overlay datalink system as disclosed in claim 50, wherein the transponder isselected from the group consisting of a TCAS transponder and an ADS-Btransponder.
 66. An overlay data link comprising: a first modulator, thefirst modulator configured to modulate a first data stream data into afirst modulated signal through a first modulation protocol; a secondmodulator coupled to the first modulator, the second modulatorconfigured to modulate an overlay message into the first modulatedsignal using a second modulation protocol to produce an overlaymodulated signal; a transmitter, wherein the transmitters is coupled tothe second modulator and the transmitter is configured to transmit theoverlay modulated signal; and a receiver, a first demodulator and asecond demodulator, wherein: the first demodulator is configured toextract first data stream by demodulating the overlay modulated signalwith the first modulation protocol; and the second demodulator isconfigured to extract the overlaid message from an overlay modulatedsignal.
 67. The overlay data link as disclosed in claim 66, wherein thefirst modulation protocol is pulse position modulation and the overlaymodulation protocol is phase shift keying modulation.
 68. The overlaydata link as disclosed in claim 66, wherein the first modulationprotocol is selected from the group consisting of: binary phase shiftkeying modulation (BPSK); quadrature phase shift keying modulation(QPSK); 8-phase shift keying modulation (8-PSK); differentialphase-shift keying (DPSK); DNPSK modulation, where N is an even integerand a multiple of 2; frequency shift keying (FSK); amplitude shiftkeying (ASK); quadrature amplitude modulation (QAM); orthogonalfrequency-division multiplexing (OFDM); minimum-shift keying (MSK);asymmetric phase-shift keying, (APSK); pulse position modulation (PPM);amplitude modulation (AM); frequency modulation (FM); and combinationsthereof.
 69. The overlay data link as disclosed in claim 66, wherein thefirst modulation protocol is selected from the group consisting of:modulation by amplitude characteristics, modulation by phasecharacteristics, modulation by frequency characteristics, andcombinations thereof.
 70. The overlay data link as disclosed in claim66, wherein the second modulation protocol is selected from the groupconsisting of: binary phase shift keying modulation (BPSK); quadraturephase shift keying modulation (QPSK); 8-phase shift keying modulation(8-PSK); differential phase-shift keying (DPSK); DNPSK modulation, whereN is an even integer and a multiple of 2; frequency shift keying (FSK);amplitude shift keying (ASK); quadrature amplitude modulation (QAM);orthogonal frequency-division multiplexing (OFDM); minimum-shift keying(MSK); asymmetric phase-shift keying, (APSK); pulse position modulation(PPM); amplitude modulation (AM); frequency modulation (FM); andcombinations thereof.
 71. The overlay data link system as disclosed inclaim 66, wherein the second modulation protocol is selected from thegroup consisting of: modulation by amplitude characteristics, modulationby phase characteristics, modulation by frequency characteristics, andcombinations thereof.